1. Field of the Invention
The present invention relates to a transmission power control method and a mobile communication system for controlling a transmission power of a transmission acknowledgement channel for uplink user data, which is transmitted from a cell controlled by a radio base station to a mobile station.
2. Description of the Related Art
In a conventional mobile communication system, when setting a Dedicated Physical Channel (DPCH) between a mobile station UE and a radio base station Node B, a radio network controller RNC is configured to determine a transmission rate of uplink user data, in consideration of hardware resources for receiving of the radio base station Node B (hereinafter, hardware resource), a radio resource in an uplink (an interference volume in an uplink), a transmission power of the mobile station UE, a transmission processing performance of the mobile station UE, a transmission rate required for an upper application, or the like, and to notify the determined transmission rate of the uplink user data by a message of a layer-3 (Radio Resource Control Layer) to both of the mobile station UE and the radio base station Node B.
Here, the radio network controller RNC is provided at an upper level of the radio base station Node B, and is an apparatus configured to control the radio base station Node B and the mobile station ET.
In general, data communications often cause burst traffic compared with voice communications or TV communications. Therefore, it is preferable that a transmission rate of a channel used for the data communications is changed fast.
However, as shown in FIG. 1, the radio network controller RNC integrally controls a plurality of radio base stations Node B in general. Therefore, in the conventional mobile communication system, there has been a problem that it is difficult to perform fast control for changing of the transmission rate of uplink user data (for example, per approximately 1 through 100 ms), due to the increase of processing load and processing delay in the radio network controller RNC.
In addition, in the conventional mobile communication system, there has been also a problem that costs for implementing an apparatus and for operating a network are substantially increased even if the fast control for changing of the transmission rate of the uplink user data can be performed.
Therefore, in the conventional mobile communication system, control for changing of the transmission rate of the uplink user data is generally performed on the order from a few hundred ms to a few seconds.
Accordingly, in the conventional mobile communication system, when burst data transmission is performed as shown in FIG. 2A, the data are transmitted by accepting low-speed, high-delay, and low-transmission efficiency as shown in FIG. 2B, or, as shown in FIG. 2C, by reserving radio resources for high-speed communications to accept that radio bandwidth resources in an unoccupied state and hardware resources in the radio base station Node B are wasted.
It should be noted that both of the above-described radio bandwidth resources and hardware resources are applied to the vertical radio resources in FIGS. 2B and 2C.
Therefore, the 3rd Generation Partnership Project (3GPP) and the 3rd Generation Partnership Project 2 (3GPP2), which are international standardization organizations of the third generation mobile communication system, have discussed a method for controlling radio resources at high speed in a layer-1 and a media access control (MAC) sub-layer (a layer-2) between the radio base station Node B and the mobile station UE, so as to utilize the uplink radio resources effectively. Such discussions or discussed functions will be hereinafter referred to as “Enhanced Uplink (EUL)”.
Referring to FIG. 3, the mobile communication system, to which the “Enhanced Uplink” is applied, is explained.
As shown in an example of FIG. 3, in the mobile communication system, a cell, which is controlled by a radio base station Node B, is configured to transmit an “Enhanced HARQ Acknowledgement Indicator Channel (E-HICH)” which is a transmission acknowledge channel, in order to perform a retransmission control of the uplink user data, i.e., “Hybrid Automatic Repeat Request (HARQ)”.
In other words, in the above mobile communication system, the cell, which is controlled by the radio base station Node B, is configured to perform an error detection check (“Cyclic Redundancy Check” CRC) of the uplink user data transmitted via an “Enhanced Dedicated Physical Data Channel (E-DPDCH)”, and to notify an “ACK” or a “NACK” to the mobile station UE using the E-HICH, so as to perform the retransmission control of the uplink user data of the mobile station UE.
To be more specific, as shown in FIG. 3, the mobile station UE, which has transmitted an E-DPDCH #1 to a cell #2, is configured to transmit the subsequent E-DPDCH to the cell #2, upon receiving of an E-HICH #1 (ACK) from the cell #2.
On the other hand, the mobile station LIE is configured to retransmit the E-DPDCH #1 to the cell #2, upon receiving of the E-HICH (NACK) from the cell #2.
Further, in the above mobile communication system, a closed loop transmission power control using a “Transmit Power Control (TPC) command” is known, as an example of the transmission power control method for a downlink dedicated physical channel (herein after, DPCH) transmitted from the radio base station Node B.
Referring to FIG. 4A, the closed loop transmission power control using the TPC command is described.
As shown in FIG. 4A, the mobile station UE, which has received a downlink DPCH transmitted from the cell #2, is configured to determine the increase/decrease of a transmission power of the downlink DPCH in the cell #2 controlled by the radio base station Node B, based on the transmission power of the received downlink DPCH. Then, the mobile station UE is configured to transmit the determined result of the increase/decrease of the transmission power of the downlink DPCH to the cell #2, using the TPC command (for example, UP command/Down Command).
In addition, the cell #2 is configured to control the transmission power of the downlink DPCH to be transmitted to the mobile station UE, using the TPC command transmitted from the mobile station UE.
In addition, in the above mobile communication system, the cell #2 is configured to determine the transmission power of the E-HICH, based on the transmission power of the downlink DPCH and a predetermined offset (an E-HICH offset).
As described above, in the mobile communication system, the reception power of the downlink DPCH in the mobile station UE will be improved by the transmission power control using the TPC command, and therefore, the reception power of the E-HICH, which depends on the downlink DPCH, will be also improved.
Next, referring to FIG. 4B, the transmission power control using the TPC command in the mobile communication system in which soft-handover (SHO) is performed is described.
In the above mobile communication system, as shown in FIG. 4B, when the mobile station UE is performing the SHO by establishing radio links with the cell #3 as well as the cell #4, and when the mobile station UE receives the same DPCHs #1 transmitted from the cell #3 and the cell #4, the mobile station UE is configured to combine the DPCH #1 received from the cell #3 and the DPCH#1 received from the cell #4, so as to determine the increase/decrease of the transmission power of the DPCH #1 in both of the cell #3 and the cell #4, based on the reception power of the combined DPCH #1.
Then, the mobile station UE is configured to transmit the determined result of the increase/decrease of the transmission power of the DPCH #1 to the both of the cell #3 and the cell #4, using the TPC command.
In addition, in the above mobile communication system, the transmission power of the E-HICH #1 transmitted from the cell #3 is configured to be determined, based on the transmission power of the DPCH #1 transmitted from the cell #3 and the predetermined offset (the E-HICH offset).
In addition, the transmission power of the E-HICH #2 transmitted from the cell #4 is configured to be determined, based on the transmission power of the DPCH #1 transmitted from the cell #4 and the predetermined offset (the E-HICH offset).
Further, as shown in FIG. 4B, in the above mobile communication system, if the mobile station UE is performing the SHO by establishing the radio links with the cell #3 as well as the cell #4, and if the reception power of the DPCH #1 transmitted from the cell #3 is good enough, even when the reception power of the DPCH #1 transmitted from the cell #4 is insufficient, the reception power of the combined DPCH #1 will be sufficient for the mobile station UE.
Therefore, in the above mobile communication system, the mobile station UE can receive the DPCH #1, if the reception power of the DPCH #1 transmitted from the cell #3 is good enough, even when the reception power of the DPCH #1 transmitted from the cell #4 is insufficient.
Accordingly, in such a condition, the transmission power of the DPCH #1 does not have to be increased, and the mobile station UE is configured not to transmit the TPC command (for example, UP command) for increasing the transmission power of the DPCH #1 transmitted from the cell #4.
However, in the above condition, as shown in FIG. 5, the transmission power of the E-HICH #2 transmitted from the cell #4 is depending on the transmission power of the DPCH #1 transmitted from the cell #4, so that the reception power of the E-HICH #2 will be insufficient, when the reception power of the DPCH #1 transmitted from the cell #4 is insufficient in the mobile station UE.
Therefore, in the above mobile communication system, when the mobile station UE is performing the SHO by establishing the radio links with the cell #3 as well as the cell #4, the mobile station UE can receive the E-HICH #1 transmitted from the cell #3, however the mobile station UE cannot receive the E-HICH #2 transmitted from the cell #4.
Accordingly, in the case as shown in FIG. 5, there has been a problem that the mobile station UE can receive the E-HICH #1 (NACK) and the mobile station UE can not receive the E-HICH #2 (ACK), even when the cell #3 transmits the E-HICH #1 (NACK) in response to the E-DPDCH #1 transmitted from the mobile station 13E, and the cell #4 transmits the E-HICH #2 (ACR) in response to the E-DPDCH #1 transmitted from the mobile station UE.
Therefore, in such a case, the mobile station UE does not transmit the subsequent uplink user data, but the mobile station UE retransmits the E-DPDCH #1 to the cell #4 unnecessarily.